MicroRNAs are small RNA molecules expressed in cells that regulate gene expression by base-pairing to mRNAs. MicroRNAs have been implicated in a variety of physiologically significant pathways, including pathways involved in pathogenesis. Most recently, microRNAs have been shown to potentially be active in defending cells against viral infections. In the current proposal, we we will obtain RNA samples from cells infected with a variety of pathogens, and attempt to determine whether or not there are specific microRNAs whose expression is modulated by infection or pathogenesis. Because of the relatively limited scope of this proposal, we have focused on using standard techniques and approaches to determine as much as possible about microRNA responses during pathogenesis. However, if individual microRNAs are identifies whose expression changes generally during pathogenesis or specifically in response to a given pathogen, then it may be possible to develop technologies that use these microRNAs as diagnostic markers or that will allow them to be introduced as prophylactics or therapeutics. In greater detail, our Specific Aims are to: 1. Develop methods for extracting microRNAs from cells and generating array data on microRNA expression. 2. Apply these methods to the detection of changes in microRNA expression during HIV infection. 3. Apply these methods to the detection of changes in microRNA expression during influenza infection. 4. Apply these methods to the detection of changes in microRNA expression with other infectious agents.